Neointima formation is a key event in both atherosclerosis and restenosis after angioplasty. Although a strong association exists between leukocyte and neointima formation, the molecular mechanisms involved in this process remain unclear. Myeloperoxidase (MPO) is a leukocyte-derived heme protein. Under physiological conditions, MPO catalyzes the reaction between hydrogen peroxide (H2O2) and chloride resulting in the formation of MPO-derived chlorinating products. High levels of MPO and MPO-derived chlorinating products are found in human atherosclerotic and restenotic lesions and that elevated levels of MPO are associated with the presence of coronary heart disease. We have recently shown that MPO has a strong negative effect on nitric oxide (NO) signaling both by direct NO consumption and inhibition of NO formation via its chlorinating products. We have demonstrated that MPO is a transcytosable protein. Vessel bound MPO can remain in the vascular wall for a significant period of time. Interestingly, vessel bound MPO not only can use leukocyte-derived H2O2, but also vascular non-leukocyte-derived H2O2 to inhibit NO signaling and induce endothelial dysfunction. Since NO is a known key regulator of neointima formation, we hypothesize that MPO contributes to neointima formation after angioplasty by diminishing NO signaling. Our preliminary data have shown, for the first time, that overexpression of MPO increases, whereas inhibition of endogenous MPO decreases, neointima formation after angioplasty. Studies in specific aim 1 will further determine the role of MPO on neointima formation in rat carotid arteries after balloon injury. Specific aim 2 will identify the molecular mechanism(s) involved in the MPO-induced effect on neointima formation in rat carotid arteries after balloon injury. Specific aim 3 will determine the effect of MPO deficiency on NO signaling and neointima formation, and the effect of NO signaling deficiency on MPO-induced neointima formation after angioplasty in gene knock-out mice. The current proposal will identify a novel mediator, leukocyte-derived MPO, in neointima formation and its molecular mechanism(s). Blocking the MPO pathway might be a new therapeutic approach to restenosis after balloon angioplasty.